The present invention relates to a process for producing a novel polyester polyurethane from a polyester polyol and a polyisocyanate. More particularly, it relates to a process for producing a polyester polyurethane which has an excellent hydrolysis resistance and keeps its good flexibility under a low temperature atmosphere.
Heretofore, polyurethanes have been produced from a high-molecular weight polyol and a polyisocyanate and, optionally, a low-molecular weight compound having two or more active hydrogen atoms. The high-molecular weight polyol includes, for example, polyester polyols and polycaprolactone polyols. The former are formed by the polycondensation reaction of a dicarboxylic acid such as an aliphatic dicarboxylic acid including succinic acid, glutartic acid, adipic acid, azelaic acid or sebacic acid with a glycol such as ethylene glycol, propylene glycol, 1,4-butanediol neopentyl glycol or 1,6-hexanediol (See "Polyurethane Resin", pp. 56-61, by Keiji Iwata, published by Nikkan Kogyo Shinbunsha, July 30, 1975).
When the above-mentioned polyester polyol or polycaprolactone polyol is used as the high-molecular weight polyol, the resulting polyurethane is poor in hydrolysis resistance, and therefore, it has a sticky surface or cracked surface when it is made into film. Because of this poor property, the applications of such a polyurethane are considerably limited. The hydrolysis resistance of the polyurethane can be effectively improved by reducing the amount of the ester groups of the polyester polyol residues in the polyurethane. This is achieved by using as the high-molecular weight polyol a polyester polyol obtained from a glycol having a large number of carbon atoms and a dicarboxylic acid. A polyurethane obtained from such a polyol is improved in hydroylsis resistance, but has a high degree of crystallinity which leads to an extremely decreased flexibility. In other words, it is very poor in flex resistance in an atmosphere at low temperature, e.g., -20.degree. C.